what determines atom attraction in chemistry?

[Chooco]

lets take a simple alkyl group, H3C-CH2-CH2-CH3
now if we add something like chlorine we could get H3C-CHCl-CHCl-CH3 + H2
those hydrogens were kicked out and replaced by chlorines, but why?

lets take these chemicals mixing: HCl + NaOH
it's a neutralization......but both HCl and NaOH were stable on their own so why did they react?
yes i know that they break apart into H + Cl + Na + OH in water.....but why would the H be stronger attracted to the OH than the Cl?

 

[rbhawcroft]

Its a while since I did chemistry but if I remeber correctly I think you should read up on the joy that is entropy. Which is basically atoms arrange in different circumstances according to the least number of possible variations or sth like that, the corollary is that just because a reaction could take place on a molecular theorey level doesnt mean it will until the entropy reaction permits it as well, for example ice only melts at about +4 Degrees Cel even though there is enought molecular energy to keep the H2O atome apart at 0 Degrees Celcius, and it also explians why reactions can be done at a lower temerature under pressure. One of the main reasons for violent reactions if I remeber is that energy (again seen as entropy) can be stored in the 'bonds' that are within compounds and that some 'bonds' have less entropy than others, therefore some reactions require energy put into them overall, eg to create N2 and others net release energy eg CHx and O2 - CO2 and H2O. Youll need to review a good source as its fundamental chemistry.

 

[rbhawcroft]

i think i missed your point a bit, if you look at the quantum level you dont have an atom you have a proton/neutron mass: the neuclei, and the free electrons, no of electrons equals protons in an atom, in a 'bond' all that has happened is that the atoms are shared as if being held in orbit by the two neuclei, and sometimes electrons are discarded eg in a battery.

the chlorine would have bonded because the -(oh) would have wanted the extra hydrogen, afterall if there isnt a -(x) floating around where did you get the Cl from? or CL2 plus 2h20 to cl(oh) cl(oh) maybe if you bubbled cl2 through the solution. that would take engery to push the entropy high enough, and then in this arificial potential state the cl would look for a better entropy state at normal reaction temps for this environment and go to the alkyl molecule.

hcl and naoh wouldnt react at conditions below reaction and entropy reaction conditions, although normal temps would probably be enough for the reaction, the entropy would be lower in this output, so when the entropy and temp is right the reaction takes place, thats why not all reagents react, normally, because with decreasing concentration of reagents and increasing concentration of reactants the entropy for the reaction goes up, its to do with the optimal numbers of atom arrangements in a temp and pressure environment.

I cant really remember any more. except that it would be inconceivable for n + cl + na and oh to be just floating about in water, random like, the key think is that water eases a reaction, its a polar molecule ie the electron denisty is greatest near the h neuclei which have a strong pull, and least near the low pulling oxygen neuclei, so you have a relative +ve field at the o neuclei and a -ve field at the h neuclei (which is just a proton and no neutron normally). the water normally breaks into a h and 2 electrons and a +(OH) molecule which is good at binding with metals which are normally long on electrons. its five years since i did alevel chemistry so some of this is a bit rusty.

 

[rgwalt]

In the acid/base reaction, you need to remember that the HCl solution has a much higher H+ concentration than for neutral water. The same is true for the OH- concentration in the NaOH solution. Now, when you mix the two, the large amount of H+ ions will react with the large amount of OH- ions. The H+ and OH- have already dissociated from Cl- and Na+ in solution, and solutions will seek a neutral PH, so the reaction occurs. The reason that the H+ reacts with the OH- instead of the Cl- is electron affinity. O is higher in the periodic table than Cl- so the last octet is closer to the nucleus. The O in the OH- is simply trying to complete its octet and will do more quickly because the last octet is closer to the nucleus than in chlorine. Also, H+ has a much greater need to bond with something than Na+, so Na+ is left to float in solution with Cl-.

croft's comments on entroy aren't completely correct... when you look at the reaction of H+ + OH- -> H20, entropy favors the left hand side as it is the higher state of disorder. However, energetically, the left hand side is favored. This is why we see some dissociation in water no matter what.

The same thing can be said for your alkane chain reaction. The products are favored energetically (they are the lower energy state).

Ryan

 

[Belegost]

One of the major determinations in the displacement of hydrogen by chlorine is electronegativity for covalent bonding (as in the alkyl) and electron affinity in the case of ionic bonding. (as in the case of NaCl) Electronegativity and electron affinity are very similiar, they quantify the amount of attraction an element has for electrons, either covalently or ionically shared. Chlorine has one of the greatest magnitudes for both of these values (I use magnitude because electron affinity is generally a negative value with the lower number implying stronger attraction.) The electron affinity for Chlorine comes in at -348.6 KJ/mol and the electronegativity is 3.0 (the highest being Fluorine at 4.0) The other consideration is Ionization Energy, the energy required to remove an electron from an atom, this value is very high for Chlorine, meaning that it does not tend to lose electrons. However, this value is extremely low for sodium

The units of KJ/mol should help clarify the meaning of the value, it is a measure of the energy released when a chlorine atom gains an electron in an ionic bond. Electronegativity isn't as simple, but it has a similiar relationship to energy and covalent bonding. So we bust out with our Gibb's Free Energy (DeltaG = DeltaH - T*DeltaS) DeltaG is the change in free energy, DeltaH is the enthalpy of reaction, T is temperature and DeltaSis the change in entropy. For a reaction to occur spontaneously DeltaG must be a negative value. So we look at the partial reactions that occur for the neutralization:

HCl ionizes to H+ and Cl-: The DeltaH here is a positive value, it takes energy to break bonds, however the DeltaS is positive since twice as many particles are produced increasing the entropy significantly. So we see for a high enough temperature HCl will ionize.

NaOH dissociates to Na+ and (OH)-: Again, DeltaH is positive here, and DeltaS is positive, same considerations as before.

H+ and OH- form H2O: DeltaH is negative here, the creation of these bonds releases energy as heat. DeltaS is a negative, this takes particles away, lowering the randomness in the system. So this will only occur for sufficiently low temperatures, in this case room temp is low enough. |T*DeltaS|<|DeltaH|

Na+ and Cl- forming NaCl: DeltaH is again negative here. DeltaS is also negative, theoretically for low enough temperatures this will occur. (though water will freeze before it does, even with the siginifcant freezing point depression form the ions.) But as noted Na and Cl tend to remain in solution, the entropy term is more than the energy can overcome at room temperature. This is mostly because Na and Cl combine in a very orderly lattice structure, which greatly decreases disorder in the system. If in solution the Na and Cl remain ionized, and it is only when the water is removed(by boiling or evaporation) that the change in entropy is overcome.

The considerations for the alkyl reaction are much the same, but the reason for the displacement is the high elctronegativity of chlorine compared to Hydrogen, which means it more readily bonds, and releases energy in the process. Furthermore, because there are the same number of particles on either side of the equation the change in entropy is negligible overall.